The effects of Mid-Holocene foragers on the European oyster in Denmark

Abstract

Oysters (Ostreidae) play a pivotal role in the health and productivity of marine ecosystems. Their unique ability to filter water, provide habitat, and contribute to nutrient cycling has remained underused in many parts of Europe following the destruction of vast oyster beds in the 19th and 20th centuries. The burgeoning field of oyster restoration for aquaculture has recognized the potential of these bivalves in promoting ecosystem resilience and enhancing biodiversity. Restoring oysters to previous levels requires the establishment of ecological baselines that ideally take into account the long-term changes of animal behavior as well as the surrounding environment prior to significant human intervention, an extremely challenging task. Archaeological shell middens are invaluable baseline archives and provide exclusive insights into past ecosystems. Here, we use demographic information from over 2,000 analyzed European oyster (Ostrea edulis) shells dating from ~5,660 to 2,600 cal BCE (calibrated years BCE), the largest archaeological growth rate dataset of mollusks yet. Through the analysis of size as well as ontogenetic age, we decouple anthropogenic from environmental impacts throughout Denmark. Our data show definitive influence of oyster size-age structure through human harvesting during the Mid-Holocene, with older oysters in the Mesolithic (mean: 4.9 y) than the Neolithic (mean: 3.7 y), irrespective of changes in growth rate. Furthermore, we present the metrics for long-term sustainable harvesting of oysters across environmental and socioeconomic transitions, providing demographic targets for current oyster restoration projects and valuable context in mitigating the impact of modern climatic change.

Keywords: archaeological shellfish; hunter-gatherer-fishers; paleoecology; prehistoric baseline; southern Scandinavia.

Fig. 1.

The natural shell bank (i.e., shell deposit at Tybrind Vig) and archaeological sites sampled in this study, including those with Middle–Late Mesolithic (circle) or Late Mesolithic and Early Neolithic (square) oysters. The inset shows one of the trenches through the eponymous Ertebølle shell midden (base-map by Vemaps, https://vemaps.com/denmark/dk-05).

Fig. 2.

(A) Size distribution by period and archaeological site. Length of oysters calculated based on hinge measurements. (B) Age distribution by period and archaeological site. Stars (*) indicate significant differences between Mesolithic and Neolithic distributions following Mann–Whitney U tests with increasing significance for each additional star (*P < 0.05, **P < 0.01, and ***P < 0.001); “ns” indicates no significant difference.

Fig. 3.

Growth curve (A) and residuals (deviations from the growth curve) (B) at specific ages. Measured in mm distance from the growth curve at the time of death with negative values indicating smaller than usual shells and positive values indicating larger than usual shells. (C) and (D) show the residuals in mm from the general growth curve by period and site, respectively, with “ns” indicating no significant differences between Mesolithic and Neolithic distributions following Mann–Whitney U tests.

Fig. 4.

Comparison of relative weights.

Fig. 5.

Stratigraphic sequence of hinge, age, and residual data. (A–C) Krabbesholm II. (D–F) Eskilsø SØ. Mesolithic and Neolithic layers are shown in green and blue, respectively.